Attenuation measuring device using an optical maser



y 967 w. w. RIGROD 3,322,026

' ATTENUATION MEASURING DEVICE USING AN OPTICAL MASER Filed Oct. 21,1955 CAL/BRATED F/G l A TTENUA TOR p m/0 F/LL 15 /0 l4 l2 Q s FL 1 DET.

T {a {I} 1 1* MASER MATERIAL ABSORPTION CELL BALANCED WEDGE ATTENUATOR0F LOSSV DIELECTRIC 23 A 22 OPT/C AX/S F/G .3 BALANCED WEDGE ATTENUATORCOMPOSED OF TWO FLU/D CELLS INVNTOR W. W. RIG/POD ATTORNEV United StatesPatent ATTENUATION MEASURING DEVICE USING AN OPTICAL MASER William W.Rigrod, Millington, NJ., assignor to Bell Telephone Laboratories,Incorporated, New York, N.Y., a corporation of New York Filed Oct. 21,1963, Ser. No. 317,698 Claims. (Cl. 88-14) ABSTRACT OF THE DISCLOSUREThe use of an external-mirror maser as a sensitive absorption cell isdescribed. Because a maser is a high-Q device, its power output issensitive to small changes in cavity losses. Hence, it is well suited tothe measurement of small absorption losses, as occur in gases andvapors, and in some liquids. Absorption measurements of a test sampleplaced within the maser cavity are made by noting the change in thesetting of a calibrated attenuator required to maintain the same maseroutput when the test sample is removed from within the cavity. Abalanced wedge attenuator of uniform optical thickness is alsodescribed.

This invention relates to optical masers and, more particularly, to theuse of such masers as sensitive absorption measurement devices.

The advent of the optical maser, or laser, has made possible thegeneration and amplification of coherent electromagnetic wave energyover a frequency range which, for the purposes of this specification, istermed the optical frequency range and which extends from the farinfrared region of the spectrum, through the visible, and through theultraviolet region.

In addition to its usefulness as a signal generator, the maser is anextremely useful instrument of measurement in a region of the frequencyspectrum that was not, heretofore, readily accessible.

Because a maser is a high-Q device, its power output is sensitive tosmall changes in resonator losses. It is, accordingly, well suited tothe measurement of small absorption losses, as occur in gases and vaporsand in some liquids.

It is, therefore, an object of this invention to utilize a maser as anabsorption measuring device.

In accordance with the invention, absorption measurements are made usingan external-mirror maser and a balanced attenuator. The sample to bemeasured is placed within the maser cavity and the resulting attenuationcompared with that produced by a calibrated attenuator. To maintainuniform attenuation over the entire maser beam width and uniform beamdeflection for all attenuator settings, a novel balanced wedgeattenuator of uniform optical thickness is used.

These and other objects and advantages, the nature of the presentinvention, and its various features, will appear more fully uponconsideration of the various illustrative embodiments now to bedescribed in detail in connection with the accompanying drawings, inwhich:

FIG. 1 is an illustrative embodiment of a maser absorption measuringdevice in accordance with the invention;

FIG. 2 is a balanced wedge attenuator of lossy dielectric material; and

FIG. 3 is a balanced wedge attenuator composed of two fluid cells.

Referring to FIG. 1, there is shown an attenuation measuring devicecomprising an external-mirror maser oscillator which includes within themaser cavity an absorption cell and a calibrated attenuator. Morespecifically the maser cavity is formed by the pair of spaced mirrorsand 11 of which mirror 10 is partially trans- 3,322,626 Patented May 30,1967 missive to permit the abstraction of energy for externalutilization. In FIG. 1 mirror 10 is a planar mirror and mirror 11 isshown as a concave mirror. These, however, are merely illustrative. Anycombination of mirrors commonly used in the maser art can be employed.

Disposed within the cavity thus defined, is an active element housingthe maser material. In FIG. 1, and for purposes of illustration, a tube12 containing a directcurrent (D.C.)-excited plasma column is used asthe active element. The particular material that is used depends uponthe frequency at which measurements are to be made. In the instant case,measurements were made at 1.153 microns using a mixture of helium andneon. (For a more detailed discussion of the helium-neon gas maser seethe copending application of A. Javan, Ser. No. 277,- 651, filed May 2,1963.) Similarly, other than D.C. pumping means for producing apopulation inversion in the maser material can be employed, as is wellknown in the art.

To minimize reflections, the ends of tube 12 are inclined at theBrewster angle.

Located within the maser cavity between tube 12 and mirror 11 is acylindrical absorption cell 13 of length L, within which the fluid to bemeasured is contained. To minimize reflections from cell 13, its endsalso are inclined at the Brewster angle.

Located between mirror 10 and tube 12 is a calibrated attenuator 14, thedetails of which will be described in greater detail hereinbelow.

Located external to the maser, and adjacent to mirror 10, is a radiationdetector 15 for indicating the presence of electromagnetic radiationfrom the maser. Detector 15 can be of the simplest construction sinceall that it need be capable of doing is indicating when the output fromthe maser is at a specified level. Typically, a phototube or athermopile is used as the detector. Advantageously, a bandpass filter,tuned to the frequency of the maser, is placed at the input to thedetector to screen the latter from any spurious radiation.

To make a measurement, the gas or liquid to be measured is placed withinthe absorption cell 13 and the maser actuated. The calibrated attenuator14 is set at some low value so as to give a convenient reading on thedetector 15. The absorption cell is then emptied of the test sample andeither filled with dry nitrogen or some other low-loss gas or,preferably, the absorption cell is evacuated. The attenuator is thenreadjusted so as to give the same output reading on the detector. Thedifference in the attenuator readings With the test sample in theabsorption cell and with the absorption cell emptied, is the absorptionof the sample material per length L. The absorption per unit length canthen be computed.

The calibrated attenuator 14- utilized in the abovedescribed measurementmust be capable of attenuating the optical beam uniformly over itsentire width. In addition, since the attenuator is inclined at theBrewster angle to minimize reflections, the beam is displaced as itpasses through the attenuator. Accordingly, the attenuatoradvantageously should be capable of displacing the beam uniformly aconstant amount regardless of attenuator setting. In accordance with theinvention, a pair of wedge attenuators, of the type shown in FIG. 2, areused to achieve these results.

Each of the elements 20 and 21 comprises two wedgelike sections which,together, form a rectangular parallelepiped. One of the wedge-likesections, such as section 22 of element 20, is a low-loss materialwhereas the other section 23 is a high-loss material. Both wedges havesub stantially the same index of refraction at the operating frequency.

Element 21 is of the same construction comprising a low-loss portion 24and a high-loss portion 25. Individual elements of the type describedare commercially available.

The two elements and 21 are placed contiguous to each other, in themanner shown, such that when their adjacent surfaces are in mutualcontact over their entire surface areas, the thick end of the high-losswedge 23 of element 20 is adjacent to the narrow end of the high-losswedge 25 of element 21. This is the condition for maximum attenuation.It will be noted that by using two wedges in the manner described, thetotal thickness of lossy material is the same for any ray passingthrough the attenua tor. Hence, the attenuation is uniform over theentire beam width. Furthermore, the total thickness of material,including both the low-loss and the high-loss portions, is the same forall settings of elements 20 and 21. Thus, the two conditions, uniformattenuation and constant displacement of the optical beam are satisfied.

The attenuation is varied by displacing one or both of the elements withrespect to each other by sliding them in a direction parallel to theircommon interface as indicated by the arrows.

A second embodiment of the balanced attenuator, shown in FIG. 3,comprises two trough-like cells and 31 with Brewster-angle windows. Thecells are filled with a material of known attenuation. As before, theattenuation is varied by sliding one or both of the identicalsymmetrical wedges parallel to their common interface. Because of thesymmetry of the two cells, there is no variation in the lateraldisplacement of the beam with changes in the attenuator setting.

In all cases it is understood that the above-described arrangements areillustrative of only a small number of the many possible specificembodiments which can represent applications of the various principlesof the invention. For example, instead of using Brewster angle windowson the absorption cell, the windows can be oriented normal to the beamdirection and reflections minimized by means of an anti-reflectingcoating. Similarly, the calibrated attenuator can be oriented normal tothe incident beam and, if necessary, an anti-reflecting coating used tominimize reflections. In this latter arrangement there is nodisplacement of the beam and hence there is no restrictions on therelative indices of refraction of the lowloss wedge and the high-losswedge comprising each of the attenuator elements 20 and 21. Thus,numerous and other varied embodiments may be devised in accordance withthese principles by those skilled in the art without departing from thespirit and scope of the invention.

What is claimed is:

1. Attenuation measurement equipment comprising:

a maser including a maser material disposed within a resonant cavity;

means for housing within said cavity a sample of a material to bemeasured;

a variable calibrated attenuator disposed within said cavity;

said attenuator comprising two substantially identical wedges of lossymaterial having a common interface;

means for sliding said wedges in a direction parallel to said interface;

and means for measuring the output from said maser.

2. The equipment according to claim 1 wherein;

said attenuator comprises two trough-like cells;

and means for filling said cells with a fluid of known attenuation.

3. The equipment according to claim 1 wherein;

said attenuator comprises two rectangular parallelepiped elements eachof which comprises a lowloss wedge section and a high-loss wedgesection.

4. Attenuation measuring equipment comprising:

an optical maser including a resonant cavity;

a sample of material to be measured disposed within said cavity;

a variable calibrated attenuator disposed within said cavity andoriented at the Brewster angle with respect to the maser beam;

said attenuator comprising two substantially identical rectangularparallelepiped elements each of which includes a wedge of low-lossmaterial and a wedge of high-loss material;

said elements being in contact with each other over a portion of theiradjacent surfaces;

means for sliding said wedges with respect to each other in a directionparallel to said surfaces;

and means for measuring the output from said maser.

5. A balanced attenuator comprising two substantially identicalrectangular parallelepiped elements each of which includes a wedge oflow-loss material and a wedge of high-loss material;

said elements being in contact with each other over a portion of theiradjacent surfaces with the thick end of the high-loss wedge of each ofsaid elements adjacent to the narrow end of the high-loss wedge of theother of said elements;

and means for sliding said wedges with respect to each other in adirection parallel to said surfaces.

References Cited UNITED STATES PATENTS 7/1954 Davidson et al 88-14

1. ATTENUATION MEASUREMENT EQUIPMENT COMPRISING: A MASTER INCLUDING AMASER MATERIAL DISPOSED WITHIN A RESONANT CAVITY; MEANS FOR HOUSINGWITHIN SAID CAVITY A SAMPLE OF A MATERIAL TO BE MEASURED; A VARIABLECALIBRATED ATTENUATOR DISPOSED WITHIN SAID CAVITY; SAID ATTENUATORCOMPRISING TWO SUBSTANTIALLY IDENTICAL WEDGES OF LOSSY MATERIAL HAVING ACOMMON INTERFACE; MEANS FOR SLIDING SAID WEDGES IN A DIRECTION PARALLELTO SAID INTERFACE; AND MEANS FOR MEASURING THE OUTPUT FROM SAID MASER.